<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin:0;
            padding: 0;
        }
        #container{
            width: 960px;
            margin: 0 auto;
            text-align: center}
        #banner{width: 960px;
            margin: 0 auto;
            height: 100px;
            border: black solid}
        #nav{width: 960px;
            margin: 0 auto;
            height: 40px;
            border: black solid}
        #content{display:flex;
            width: 960px;
            margin: 0 auto;
            height: 400px;
            border: red solid}
        .content_left{
            width:320px;
            float:left;
            height:400px;
            border: red solid
        }
        .content_right{
            width:320px;
            float:right;
            height:400px;
            border: red solid
        }
        .content_center{
            flex: 1;
            height:400px;
            border: red solid
        }
        #footer{
            width: 960px;
            height:100px;
            clear: both;
            border: black solid;
            margin: 0 auto;}
        #footer2{
            width: 960px;
            height:150px;
            clear: both;
            border: black solid;
            margin: 0 auto;}
        </style>
</head>
<body>
    <div id="container">
        <div id="banner"></div>
        <div id="nav"></div>
        <div id="content">
            <div class="content_left"></div>
            <div class="content_center"></div> -+
            <div class="content_right"></div>
        </div>
        <div id="footer"></div>
        <div id="footer2"></div>
    </div>
</body>
</html>